﻿326 ME. A.. M. FINLAYSON ON OEE-BEPOSITION [May ICflO, 



impoverished. 1 Another occurrence which is probably of secondary 

 origin, is that of the plumosite and argentiferous tetrahedrite at 

 Foxdale, which were found in vughy and cellular portions of the 

 lode at depths of 600 feet," and have not been met with in the 

 deeper workings. 



An equally important secondary process has been the depletion 

 of blende from the upper parts of the veins by conversion into 

 soluble sulphate, and its subsequent deposition as calamine by the 

 replacement of limestone or dolomite. Calamine has been a 

 common ore in Flintshire veins to a depth of 200 feet, while it has 

 been the chief zinc-ore mined in North Derbyshire. The calamine- 

 deposits of the Mendip Hills in Somerset, 3 and of the Silver- 

 mines district in Tipperary, 4 have been formed by the removal of 

 the blende from the adjacent lead-veins, and the deposition of 

 calamine by replacement of the dolomite beds in which the deposits 

 occur. They are analogous to the calamine deposits of Upper 

 Silesia, South- Western New Mexico, and Virginia. 



VIII. SlTMMAEY. 



The natural history of the ores in the districts here discussed 

 may now be summarized. 



The fluorine and heavy metals of the ores have been derived from 

 a deep-seated source, probably in the first place by marginal segre- 

 gation in underlying magmas. They were carried upwards chiefly 

 by 'juvenile' waters emanating from these magmas, fluorine being 

 an important constituent in the deeper zones. In the upper 

 horizons of limestone districts, the underground circulation of 

 meteoric waters has had an important effect on the distribution and 

 deposition of the ores in the veins. The vein-solutions carried 

 alkaline sulphides, which held the metals in solution as sulphides, 

 and also alkaline and earthy carbonates. These latter compounds 

 have effected sericitization and carbonatization in the adjoining 

 country-rock, except in the case of limestones, where silicification 

 and dolomitization took place near the vein-walls. The rilling of 

 spaces rather than replacement of rock has been the chief feature 

 in the deposition of the ores, but the process of deposition has 

 itself involved interchange of constituents between rock and 

 solution, as expressed in the chemical alteration of the wall-rocks. 

 The calcium of fluorspar has been very largely derived from the 

 adjoining rocks. Of the vein-minerals, chalcopyrite was first 

 deposited, followed by fluorspar, blende, galena, and pyrite in that 

 order. Ore-deposition has persisted over a maximum vertical range 

 of about 5000 feet, chalcopyrite and blende being characteristic of 

 the lower horizons, galena and fluorspar of the upper. The country- 

 rock has influenced ore-deposition, first by its physical character 



1 M. F. Heddle, ' Mineralogy of Scotland ' Edinburgh, vol. i (1901) pp. 10-11. 



2 W. W. Smyth, Trans. Koy. Geol. Soc. Cornwall, vol. x (1887) p. 82. 



3 H. B. Woodward, ' Geology of East Somerset & the Bristol Coalfields ' 

 Mem. Geol. Surv. 187G, pp. 167 et seqq. 



1 Explan. of Sheet 134, Mem. Geol. Surv. Ireland, 1861, p. 31. 



